Laminated substrates having a heat sink are known in the art. Typically, these devices are utilized where the application of high power requires dissipating the heat generated by active components, conductor resistance losses, and dielectric losses. One such high power application is the field of microwave power amplifiers.
Prior art microwave power amplifiers are traditionally based on modules comprising an aluminum carrier plate, upon which discrete alumina substrates are selectively bonded. Thereafter, metal-cladding techniques are employed to bond thin conductive strips to the substrate for providing electrical interconnection between the discrete components, including the power transistors, which comprise the power amplifier design. Through-holes in the alumina substrate and the aluminum carrier plate allow the transistors to be thermally connected to the module's heat sink. Each prescreened (with solder paste) module is then positioned on a spring loaded carriage or fixture, designed to provide a clamping force upon the individual components as the fixture travels through a reflow oven where the solder is reflowed.
There are two particular problems with the prior art devices. First is the poor adhesion between the ceramic (alumina) substrate and the metallic (aluminum) carrier plate. Second is the intolerable mismatch between the coefficients of thermal expansion (CTE) of alumina, aluminum, and the thin conductive strips. These problems typically surface during module temperature cycling, when the differing rates of thermal expansion between alumina, aluminum, and the conductive strips cause residual strains to develop in the adhering bonds between the differing materials. The resultant forces are primarily responsible for the broken components, compromised solder joints, and over stressed device leads experienced by prior art power amplifier modules.
In response, the industry has turned to laminated thermally conductive circuit boards as a possible solution. While these devices have gained increased acceptance for overcoming many of the interconnect problems associated with the alumina substrate modules, they nonetheless suffer from shortcomings of their own. For example, extreme thermal shock or temperature cycling can cause cracks in the conductive strips on a laminate circuit board when the CTE mismatch between the thermally conductive material and the dielectric material is unaccounted for.
Yet another concern is etch shrink, the X,Y dimensional change when metallic foil is etched to produce circuit board conductive strips. Because of CTE mismatches between the thermally conductive material and the dielectric material presently employed in metal clad laminates, internal strains develop in the laminate as it cools down after the process of heat bonding metal foil to the dielectric substrate. When the foil is subsequently etched to produce circuit board conductive strips, the internal strain often causes uncontrolled dimensional changes in the conductive strip patterns, thereby rendering several laminates unusable.
In addition, the CTE mismatch between the thermally conductive material and the dielectric material will cause the laminate to bend uncontrollably under the temperatures associated with most soldering processes. While the laminate may resume its original shape after cooling, the temperature induced contortions nontheless cause residual stresses to develop in surface mount device leads and solder joints. This stress promotes the breaking of device leads and the rapid degradation of solder joints. Accordingly, the temperature induced deflection experienced by prior art laminates may result in compromised circuit board performance.
One industry solution suggests providing a thick metal ground plane for the laminate circuit board. See "Thick-Metal Cladding On RT/duroid Microwave Circuit Dielectric Laminates," Rogers Corporation., Microwave Materials Division, August 1987, 100 S. Roosevelt Ave, Chandler, AZ 85226. According to this method, the thick metal layer must be at least three times the dielectric thickness for soft metals like aluminum and copper, and two times the dielectric thickness for alloys such as brass or stainless steel. Not surprisingly, the stiffness of the thick metal layer, if thick enough, will dominate the composite effect of the laminate CTE mismatch and prevent the laminate from bending during temperature cycling. The reality is that metal clad laminates with heat sinks two and three times the thickness of the dielectric substrate are simply impracticable in many applications.
The average power transistor used in a Radio Frequency (RF) power amplifier requires a dielectric layer of at least 0.041.+-.0.005 inches thick in order to assure proper solder contact of device leads. According to thick metal cladding techniques, however, a thermally conductive layer of 0.08 or 0.12 inches will be required to assure that the laminate does not warp under temperature. These dimensions suggest a device whose total thickness is somewhere in the neiqhborhood of 0.12 to 0.16 inches. The modern trend in thick metal cladding, however, is moving away from the specialized requirements associated with the assembly of such hybrid devices towards uniformity.
The typical printed circuit board (PCB) utilized in digital, audio, low frequency, and other applications is approximately 0.062.+-.0.005 inches in thickness. The desire is to develop a metal clad laminate that rivals standard PCBs in fit and form in order to take advantage of the reduced costs associated with production in the standard PCB environment. It would be extremely advantageous therefore to provide a laminated thermally conductive substrate that conforms to PCB industry standards, and experiences minimal deflection during temperature cycling.